This invention relates generally to solid propellant processing and in particular to processing extruded double base propellants.
Double base propellants are homogenous propellants containing a first energetic combustible plasticized by a second energetic combustible or a mixture of additional combustibles to form a plasticized energetic binder. Throughout this specification the first energetic combustible is referred to as the energetic binder, the second energetic combustible as the energetic plasticizer, and their combination as the plasticized energetic binder.
Oxidizers and metal fuels may be added to double base propellants. If an additional oxidizer(s) is added, the double base propellant is referred to as a modified double base propellant. A double base propellant with an additional oxidizer(s) and an additional metal fuel(s) is referred to as a composite modified double base propellant. Such propellant may have solids loading as high as 75 weight percent.
The term "plasticization" is synomous with gelatinization. It describes the initial reaction of the energetic binder with the energetic plasticizer. By this reaction, the binder and the energetic plasticizer form a soft colloidal dispersion.
In order to increase the mechanical strength of a propellant and prevent shape distortion during long storage, the plasticized energetic binder is crosslinked with a polyisocyanate compound which is termed the "crosslinker". The crosslinker is prepared by reacting two moles of a difunctional isocyanate with one mole of a difunctional hydroxy terminated material. To improve the crosslinking, a crosslinking catalyst is added.
The mechanism by which the freshly mixed propellant becomes usable for propellant purposes is termed "cure". The mechanism encompasses both the setting of the soft colloidal dispersion of the energetic binder and energetic plasticizer and the crosslinking of the crosslinker with the plasticized energetic binder. By curing the propellant becomes a tough elastic rubbery substance.
Until now the only known way of manufacturing crosslinked double base propellants has been the cast method. The major drawbacks of this method are the large tooling expenses, high labor costs, air entrapment problem, and worker exposure to explosions.
The importance of crosslinked double base propellants and the disadvantages associated with the cast method of preparation have generated a great interest in finding another method. Extrusion would be an excellent alternate method. The method would not require the use of expensive tooling for long periods of time or high labor costs. The method is semi-continuous as opposed to the batch operation of the cast method. Also the extrusion method can be operated by remote control, thereby avoiding exposure of the workers to the possibility of explosions. Unfortunately due to the difficulties presented by the simultaneous mechanisms of plasticization and crosslinking, the hardness requirements for the extruding material of an extruder, success has not been obtained with extrusion methods of preparation for crosslinked double base propellants.